The invention relates to a process for the low-temperature fractionation of air.
The general principles of low-temperature fractionation of air and the design of rectifier systems for nitrogen/oxygen separation with two or more columns in particular are known from the monograph xe2x80x9cTieftemperaturtechnikxe2x80x9d (Low-Temperature Technology) by Hausen/Linde (2nd edition, 1985) or from an article by Latimer in Chemical Engineering Process (Vol. 63, No. 2, 1967, page 35). The pressure column (also commonly termed xe2x80x9chigh pressure columnxe2x80x9d in the United States) and low-pressure column of a two-column system generally exchange heat via a condenser/evaporator system (principal condenser), in which top gas from the pressure column is liquefied against evaporating bottom liquid from the medium-pressure column.
The rectifier system of the invention may be designed as a conventional two-column system, but also as a three-column or multicolumn system. In addition to the columns for nitrogen/oxygen separation, it may have further devices for obtaining other components of air, in particular noble gases, for example to obtain argon.
A heat exchanger which is designed as a condenser/evaporator has evaporation passages and liquefaction passages. A liquid is evaporated in the evaporation passages. They are in heat-exchanging contact with the liquefaction passages, in which a gaseous fraction condenses in indirect heat exchange with the evaporating liquid. Details of evaporation procedures are given, for example, in the monograph xe2x80x9cVerdampfung und ihre technischen Anwendungenxe2x80x9d [Evaporation and technical applications thereof] by Billet (1981). A condenser/evaporator may be composed of one or more heat-exchanger blocks. A condenser/evaporator system has one or more condenser/evaporators.
For decades, the low-temperature fractionation of air used almost exclusively forced circulation evaporators as condenser/evaporators. In this type of evaporator, a heat-exchanger block is arranged in a bath of the liquid which is to be evaporated. The evaporation passages are open at the top and bottom. Liquid from the bath is entrained upwards by the gas formed during the evaporation (thermosiphon effect) and flows back into the liquid bath. In this way, a natural circulation of liquid is provided purely by the evaporation operation, without mechanical energy being supplied.
For some time, falling-film evaporators have also been used as condenser/evaporators in air fractionation installations, as described, for example, in EP 681153 A or EP 410832 A. In this type of evaporator, the liquid which is to be evaporated enters the evaporation passages at the top and flows downwards as a relatively thin film along the walls which separate the evaporation passages and liquefaction passages. This type of evaporator has a particularly low pressure, loss in the evaporation passages and is therefore generally more favourable in terms of energy than a forced circulation evaporator.
However, during evaporation of an oxygen-rich liquid, total evaporation, which would lead to the evaporation passages running dry, must be prevented. For this purpose, liquid emerging from the evaporation passages is generally returned to the inlet of the evaporation passages by means of a pump. Firstly, this measure is detrimental to the energy-saving action of the falling-film evaporator; secondly, levels of undesirable constituents with a low volatility in the liquid are increased.
The invention is therefore based on the object of providing a process of the type described in the introduction and a corresponding apparatus which can be operated economically and particularly favourably in terms of operating technology and in particular have a particularly low energy consumption.
This object is achieved by the features of the characterizing part of patent claim 1. Although, as in standard falling-film evaporation, the liquid which is not evaporated in the falling-film evaporator (first section of the condenser/evaporator system), i.e. the second oxygen-rich liquid, is fed to a delivery device, for example a pump, this device does not convey the liquid back to the inlet of the evaporation passages of the same falling-film evaporator, but rather to a second section of the condenser/evaporator system. Consequently, the first section only has to carry out a relatively small part, for example 30 to 50%, preferably 38 to 42%, of the total evaporation capacity of the condenser/evaporator system. The natural proportion of liquid at the outlet of the evaporation passages of the falling-film evaporator is correspondingly high. It is thus possible to dispense completely or to a large extent with an artificial circulation of liquid. The delivery device allows the liquid which has not been evaporated for the time being to flow onwards to a second section of the condenser/evaporator system. This second section is designed completely or partially as a forced circulation evaporator, where the problem of the need for an artificial circulation of liquid does not occur, or occurs to a lesser extent.
Within the context of the invention, it has emerged that with the aid of the measures according to the invention, the volume of pumped liquid can be reduced to approximately 30%. The effect of the reduced pumping capacity on the energy balance is not restricted to the driving energy saved; rather, the benefit is based to a greater extent on the reduced introduction of heat which results from the smaller delivery volume of second oxygen-rich liquid.
In the process according to the invention, the oxygen product is preferably removed from the second section of the condenser/evaporator system, either as a gas or as a liquid. In the latter case, it is possible, if appropriate, to obtain a gaseous pressurized oxygen product in addition to a liquid oxygen product by bringing oxygen-rich liquid in the liquid state to an elevated pressure and then evaporating it against air or nitrogen (so-called internal compression).
The first section of the condenser/evaporator system of the invention may be arranged inside the low-pressure column or in a separate vessel.
The process according to the invention and the corresponding apparatus can be used for any type of nitrogen/oxygen separation, in particular independently of the purity of the products in the heads and bottoms of the columns.
The vapour which is produced in the evaporation passages of the second section of the condenser/evaporator system is preferably not exclusively or primarily removed as a gaseous oxygen product, but rather at least half of this vapour is introduced into the low-pressure column, where it is used as rising vapour. If the entire oxygen product is obtained in liquid form and/or is internally compressed it is also possible for all the gas produced in the second section of the condenser/evaporator system to be returned to the low-pressure column.
A third oxygen-rich liquid remains in the second section of the condenser/evaporator system, as the unevaporated part of the second oxygen-rich liquid. It preferably collects in the liquid bath of the or one forced circulation evaporator. In the process according to the invention, it is preferable for at least some of this third oxygen-rich liquid to be returned to the low-pressure column and/or to the evaporation passages of the first section of the condenser/evaporator system. This returning may advantageously be carried out together with the abovementioned return of vapour to the low-pressure column, as a result of a suitable line being arranged at the height of the liquid level in the bath. This at the same time regulates the liquid level in the forced circulation evaporator without additional control devices being required.
If the second section is partially designed as a second falling-film evaporator, it is additionally possible for the delivery device which is in any case present between the first and second sections additionally to be used to produce a circulation of liquid at the second falling-film evaporator.
The liquefaction passages of the condenser/evaporator system are preferably connected to the two columns in the way which is described in patent claim 4. As a result, it is possible to dispense with pumps at these locations, even if the pressure column and low-pressure column are arranged next to one another. (In this case it is advantageous if the first section of the condenser/evaporator system is arranged beneath the bottom plate of the low-pressure column and the second section of the condenser/evaporator system is arranged above the top plate of the pressure column.)
The first section, which is designed as a falling-film evaporator, is preferably dimensioned in such a way that in this evaporator, condensation of a nitrogen-rich gas fraction from the pressure column leads to the formation of the amount of nitrogen-rich liquid which is required as reflux in the low-pressure column (plus, if appropriate, the amount removed as unpressurized liquid product). This represents, for example, a proportion of 30 to 50%, preferably 38 to 42%, of the total heat-transfer capacity of the condenser/evaporator system. The remainder of the heat transfer (50 to 70%, preferably 58 to 62%), is carried out in the second section of the condenser/evaporator system, specifically in such a way that at least the amount of liquid which is required as reflux in the pressure column is produced therein.
For reasons of the spatial distribution of the heating surface, it may in some cases be more advantageous for a larger proportion of the nitrogen-rich fraction than that described above to be condensed in the first section, in order for a corresponding amount of heating surface to be displaced from the second section (generally at the head of the pressure column) to the first section (generally in the bottom of the low-pressure column). In this case, some of the first nitrogen-rich liquid which is formed in the first section is fed to the pressure column as reflux. This may require the use of a liquid pump.
The nitrogen-rich gas fraction is generally formed by head nitrogen in the pressure column.
The first section of the condenser/evaporator system is preferably designed exclusively as a falling-film evaporator. With the aid of the dimensions outlined above, it may particularly advantageously be produced as an individual, relatively compact block or in the form of a plurality of (for example four) particularly low blocks which are arranged next to one another. An arrangement directly in the bottom of the low-pressure column is also advantageous with a view to achieving a low structural height of the installation and its insulation (coldbox).
The second section of the condenser/evaporator system may be formed by at least two partial sections which are connected in series on the evaporation side and the first of which is designed as a falling-film evaporator and the second of which is designed as a forced circulation evaporator. The liquid which flows out of the evaporation passages of the partial section which is in the form of a falling-film evaporator is, for example, introduced into the liquid bath of the or one partial section which is in the form of a forced circulation evaporator. The falling-film evaporator/forced circulation evaporator combination may, for example, be equipped with continuous liquefaction passages, as described in detail in EP 795349 A. In this case, the liquid from the bath of the forced circulation evaporator may be returned to the low-pressure column or to the outlet of the evaporation passages of the first section of the condenser/evaporator system and may be used to increase the amount of liquid in that partial section of the second section which is designed as a falling-film evaporator.
The invention also relates to an apparatus for the low-temperature fractionation of air.